The objective of this proposal is to develop a method by which cardiac tissue and whole hearts can be cooled to and stored at very low temperatures, and when returned to a normal environment resume physiological activity. To accomplish this the specimens will be exposed to and perfused with gradually increasing concentrations of cryoprotective agents at gradually lower temperatures. The concentration of the cyroprotective agents will always be sufficient to depress the freezing point of the medium used below the temperature to which the specimen is exposed. Particular attention will be given to the nature and concentration of the ions incorporated into the perfusion medium when specimens are exposed to low temperatures. Attempts will be made to correlate injury or survival after cooling with: a) the exposure temperature, b) the length of exposure, c) the concentration and nature of cyroprotective agents used, d) the distribution and quantity of ice formed in the tissue, and e) ultrastructural changes in the tissue after exposure to a given treatment.